(1) Field of the Invention
The invention relates to field emission flat panel displays, and more particularly to structures and methods of manufacturing field emission displays that provide a focus mesh for such displays.
(2) Description of the Related Art
In display technology, there is an increasing need for flat, thin, lightweight displays to replace the traditional cathode ray tube (CRT) device. One of several technologies that provide this capability is field emission displays (FED). An array of very small, conical emitters is manufactured, typically on a glass substrate, and are addressed via a matrix of columns and lines. These emitters are connected at their base to a conductive cathode, and the tips of the emitters are surrounded by a second conductive surface usually referred to as the gate. When the proper voltages are applied to the cathode and gate, electrons emission occurs from the emitter tips, with the electrons attracted to a third conductive surface, the anode, on which there is cathodoluminescent material that emits light when excited by the emitted electrons, thus providing the display element. The anode is typically mounted in close proximity to the cathode/gate/emitter structure and the area in between is a vacuum.
FIG. 1 is a cross-sectional view of a portion of a field emission display. Column electrodes 12, also called the cathode, are formed on a baseplate 10, and have emitter tips 14 mounted thereon. The emitters are separated by insulating layer 16. A row electrode 18, or gate, with openings for the emitter tips, is formed on the insulating layer 16 and is formed perpendicular to the column electrodes. When electrons are emitted, they are attracted to conductive anode 22 and upon striking phosphor 25 mounted on the anode, light is emitted, which can be viewed through the transparent faceplate 24.
When electrons are emitted from emitter tip 14, they disperse as shown by lines 26. The radius of the spot size 28 is determined by the equation ##EQU1## where V.sub.gc is the cathode-to-gate voltage, V.sub.a is the anode voltage, and d is the distance 30 from the gate to the anode. Two important design considerations place opposing requirements on the gate to anode distance d. Throughput is increased by a larger d because the gas contained between the anode and cathode is easier to pump out. However, to provide higher display resolution a smaller spot size is desirable, and, given the equation above, a smaller distance d is needed.
Workers in the art are aware of these problems and have attempted to resolve them, by adding structures to the FIG. 1 display to focus the emitted electrons onto a smaller spot size. In one approach, such as in U.S. Pat. No. 5,186,670 (Doan et al.), another conductive surface called a focus ring, or focus gate, is added above, close to and parallel to the gate. Openings are formed above the emitters and above similar openings in the gate. When the proper voltage is applied to the focus ring, electrons emanating from the emitters are deflected into a collimated beam, However, this approach increases drive capacitance, thereby undesirably increasing power consumption. In addition, the local electric field (in the vicinity of the emitter tips) is reduced, leading to a reduction in emission current.
A second approach is disclosed in U.S. Pat. No. 5,225,820 (Clerc) in which focussing is effectively accomplished at the faceplate, in which there are three addressable anodes for each color pixel. By applying a high voltage to the anodes for which the phosphors are desired to be excited, the emitted electrons move only toward the desired anode. This approach also leads to increased power consumption because of the anode addressing voltage, which is usually several hundred volts, and this only improves the focus in a single direction, perpendicular to the anode strips.
A related problem in the manufacture of the anode plate of field emission displays, for color applications, has been that multiple masks are needed when forming the anode/phosphor structure. For example, U.S. Pat. No. 5,225,820 (Clerc) discloses the use of several masks to form the anode/phosphor structures (where there are three such structures for each display pixel) and the interconnecting lines by which the anode lines are addressed.